Info

Nobel and Pimienta-Barrios (1995); Pimienta-Barrios and Nobel (1998)

Data are restricted to those expressed on a unit surface area basis and under essentially optimal environmental conditions. For a more

complete coverage of the earlier literature, see Nobel (1988). Values are

for stems unless otherwise indicated.

Usually there is no net CO2 uptake during the middle of the daytime, but rather a slight loss occurs (Phase III; Fig. 4.1); at this time the acids accumulated the previous night are decarboxylated, and the CO2 released within the plant is refixed in the stems using Rubisco. The high internal concentration from this released CO2 causes the daytime leakage of CO2 from the stems. Stomates tend to reopen in late afternoon after stem temperatures have decreased, leading to net uptake and fixation of CO2 using Rubisco (Phase IV; Fig. 4.1).

Maximal instantaneous rates of net CO2 uptake are about 3 pmol m-2 s-1 for Opuntiapolyacantha, which is native to the central United States, 8 pmol m-2 s-1 for O. hu-mifusa, the mostly widely distributed native opuntia in the United States, and 18 pmol m-2 s-1 for O. ficus-indica, the most widely cultivated cactus worldwide (Fig. 4.1). Species in the largest subfamily, Cactoideae, also have net CO2 uptake by stems at night in the CAM mode. Maximal nighttime net CO2 uptake rates are high for Cereus validus, Ferocactus acanthodes, Mammillaria dioica, and Pachycereus pringlei (average about 6 pmol m-2 s-1) and Stenocereus queretaroensis (11 pmol m-2 s-1; Table 4.1). Maximal net CO2 uptake rates for epiphytic cacti, such as species of Cryptocereus, Epiphyllum, Rhipsalis, and Schlumbergera, are much lower (2-3 pmol m-2 s-1), although the cultivated, shade-tolerant, vinelike, hemiepiphytic cactus Hylo-cereus undatus can have maximal rates of 5 pmol m-2 s-1 (Table 4.1). Values for total daily net CO2 uptake under optimal environmental conditions are consistent with maximal instantaneous rates of net CO2 uptake for cacti. Among the cacti investigated, maximal total daily net CO2 uptake occurs for opuntias (698 mmol m-2 day-1 for O. ficus-indica and 506 mmol m-2 day-1 for Opuntia stricta) and S. queretaroensis (317 mmol m-2 day-1; Table 4.1).

Water-Use Efficiency

The importance of nocturnal stomatal opening and the accompanying net CO2 uptake with regard to ecological or agronomic success of cacti relates less to improving CO2 acquisition than to reducing water loss. In particular, air and stem temperatures are lower at night, which leads to a lower concentration of water vapor in the stem and hence less water loss for a given degree of stomatal opening; this is important in arid regions (less than 250 mm annual rainfall) or semiarid regions (250 to 450 mm annual rainfall). The ratio of CO2 fixed to water loss in transpiration is termed the water-use efficiency, which is an important benefit:cost index for assessing the gas-exchange performance of plants. For stems of cacti under favorable environmental conditions with predominantly nocturnal net CO2 uptake, approximately 10 mmol of CO2 is taken up per mol of water transpired over a 24-hour period. This water-use efficiency is about three times higher than for highly efficient C4 species such as Zea mays (maize) and Sac-charum officinarum (sugarcane) and five times higher than for highly efficient C3 species such as Medicago sativa (alfalfa) and Triticum aestivum (wheat; Nobel 1991).

To better understand the reason water loss is reduced at night due to lower temperatures, consider the water-vapor concentration at saturation (as essentially occurs within the stems of cacti). The saturation water vapor content of air increases nearly exponentially with increasing temperature, whereas the water content of the ambient air surrounding a plant remains relatively constant unless the weather abruptly changes. In particular, air saturated with water vapor at i5°C, which can represent nighttime temperatures, contains 12.8 g m-3; at 25°C, which can represent daytime temperatures, air saturated with water vapor contains 23.1 g m-3 (Nobel 1999). If the ambient air contains 8.0 g water m-3, which corresponds to 46% relative hu midity at 2o°C, the leaf-to-air difference in water vapor concentration (the force leading to water loss by transpiration) is 12.8 - 8.0 or 4.8 g m-3 at i5°C and 23.1 - 8.0 or 15.1 g m-3 at 25°C, which is threefold greater at the higher temperature. Therefore, transpiration would be threefold greater at a daytime temperature of 25°C than for the same degree of stomatal opening at night at i5°C. This is a key feature for the water-conserving nature of CAM used by cacti in arid and semiarid regions. In particular, because tissue temperatures in the field tend to average about io°C lower at night than during the daytime, CAM plants tend to lose only 20 to 35% as much water for the same degree of stomatal opening during the principal periods of net CO2 uptake as do C3 or C4 plants.

Net CO2 Uptake: Stems

Net CO2 uptake, which has been determined per unit surface area for about 20 species of cacti (Table 4.1), depends on several environmental factors. The three key environmental factors for drought-enduring plants are temperature, soil moisture, and the solar irradiation absorbed by photosynthetic pigments, i.e., the photosynthetic photon flux (PPF), which represents wavelengths of 400 to 700 nm. The response of net CO2 uptake by Opuntia ficus-indica, the most widely studied cactus, to these three variables (Fig. 4.2) is important for predicting its productivity under any environmental condition and serves as a model for assessing the net CO2 uptake, and hence the potential biomass productivity, of other cacti.

Temperature

The daily pattern and the magnitude of total net CO2 uptake by O. ficus-indica (Fig. 4.1) mainly reflect nocturnal temperatures. In particular, PEPCase is more important for the initial binding of CO2 than is Rubisco. The optimal mean nocturnal temperature is relatively low, i5°C (Fig. 4.2A). Moreover, substantial net CO2 uptake occurs at 0°C for O. ficus-indica, and O. humifusa can even have substantial net CO2 uptake at air temperatures of -5°C (Nobel and Loik 1990). Thus, low nighttime temperatures are not disadvantageous for net CO2 uptake by these cacti, whereas high nighttime temperatures, such as those above 30°C (Fig. 4.2A), can lead to appreciable stomatal closure and hence limited net CO2 uptake.

Although most cacti examined have a low temperature optimum (near i5°C) for net CO2 uptake, epiphytic cacti native to the tropical areas of the Americas are subject to much higher mean nocturnal temperatures for most of the year and can acclimate to higher stem temperatures for their PEPCase activity. For instance, Hylocereus undatus has

0 10 20 30

mean nighttime air temperature (°c)

0 10 20 30

mean nighttime air temperature (°c)

10 20 30 40

drought length (days)

10 20 30 40

drought length (days)

0 10 20 30

total daily ppf (mol m-2 day-1)

Figure 4.2. Influence of three environmental factors on net CO2 uptake by O. ficus-indica over 24-hour periods: (A) temperature, in particular the mean nighttime air temperature, which was generally within i°C of the temperature in the middle of the chlorenchyma; (B) water availability, measured by drought length, defined as when the stem water potential becomes less than the soil water potential adjacent to the roots, preventing plant water uptake; and (C) solar irradiation that can be absorbed by photosynthetic pigments, known as PPF (wavelengths of 400 to 700 nm, i.e. blue to red). Except when that specific factor was varied, the plants were under essentially optimal conditions of nighttime temperatures near ij°C, wet soil, and a total daily PPF of about 25 mol m-2 day-1 incident on the cladode surfaces. Data are from Nobel and Hartsock (1983, 1984, 1986b), Nobel and Israel (1994), and Israel and Nobel (1995).

0 10 20 30

total daily ppf (mol m-2 day-1)

Figure 4.2. Influence of three environmental factors on net CO2 uptake by O. ficus-indica over 24-hour periods: (A) temperature, in particular the mean nighttime air temperature, which was generally within i°C of the temperature in the middle of the chlorenchyma; (B) water availability, measured by drought length, defined as when the stem water potential becomes less than the soil water potential adjacent to the roots, preventing plant water uptake; and (C) solar irradiation that can be absorbed by photosynthetic pigments, known as PPF (wavelengths of 400 to 700 nm, i.e. blue to red). Except when that specific factor was varied, the plants were under essentially optimal conditions of nighttime temperatures near ij°C, wet soil, and a total daily PPF of about 25 mol m-2 day-1 incident on the cladode surfaces. Data are from Nobel and Hartsock (1983, 1984, 1986b), Nobel and Israel (1994), and Israel and Nobel (1995).

maximum nocturnal net CO2 uptake at 25°C (Raveh et al. 1995). Just as high temperatures have relatively little influence on the survival and distribution of cacti, high daytime air and stem temperatures are not critical for daily net CO2 uptake by cacti (Nobel 1988). Although the mean nighttime temperature is more useful for quantifying nocturnal net CO2 uptake, the minimum nighttime temper ature (usually 3°C to 6°C lower than the mean temperature) is more readily available from weather records and can be correlated with net CO2 uptake as well.

Water

As the water content of the soil declines and its water potential decreases below that of the plant, thermodynamic theory predicts that water will move from the plant to the soil (Nobel 1999). Water is also continually being lost from the shoot of a plant to the atmosphere. The inevitable loss of stem water as the soil dries inhibits cellular processes that lead to net CO2 uptake (Fig. 4.2B). In particular, little change in net CO2 uptake occurs during the first 7 days of drought for O. ficus-indica because of the water stored in its cladodes and the high water-use efficiency of the CAM pathway. Daily net CO2 uptake then halves over the next 17 days of drought and becomes near zero at 50 days (Fig. 4.2B); the gradual decline in part reflects the inherently low transpiration rates resulting from a low stomatal frequency and a thick cuticle. The response to soil water content can be used to devise irrigation schedules for O. ficus-indica and other cultivated cacti in arid and semiarid regions and to discern seasonal patterns of net CO2 uptake for native cacti in deserts.

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment